In the case of a roller mill including a grinding table which is rotatable about a vertical axis, and grinding rollers urged against the grinding table, the grinding table may rotate around the vertical mill axis with the grinding rollers being immovable in the direction of rotation. However, the rollers are movable up and down while being urged against the grinding table, for example, by means of draw bars activated by hydraulic cylinders. The roller mill is encased in a housing which is divided into a lower part under the grinding table and an upper part above the grinding table. The two parts or chambers into which the mill housing is divided, are interconnected via a nozzle ring, an annular passage of which is substantially vertically oriented. Air is directed into an air duct in the lower part of the mill housing and then up around the grinding table through the nozzle ring, the guide vanes of which direct the air into the upper part of the mill housing and over the grinding table in a direction with a component substantially tangential to the grinding table. Above the nozzle ring there is normally a conical, air guiding wall which further forces the air to flow over the grinding table.
The air passed to the upper part of the mill housing is used partly for transporting ground material from around the grinding table upwards and hence out of the mill, partly for drying the material if it is moist, and partly for cooling the ground material.
In order to avoid that material whirled around in the mill housing by the air and the rotating parts of the mill falls down on to the nozzle ring and clogs it, or even through the nozzle ring into the air duct in the lower part of the housing, comparatively high air velocities are required through the nozzle ring. However, this entails a rather large pressure drop across the nozzle ring. As relatively large unground pieces of material may fall on the nozzle ring, it will be appreciated that the air velocity necessary for keeping the nozzle ring clean and the equivalent air consumption are often larger than necessary for transporting and cooling and, if necessary, drying the ground material.
To keep the air suction energy consumption as low as possible, it is desirable that the amount of air supplied does not exceed what is required for transporting, cooling, and possible drying the ground material. When grinding already dry materials it is not even necessary to use air for drying them.
Roller mills of the kind as described above are known in which the air velocities through the nozzle ring are comparatively low, but in which, on the other hand, unground material is allowed to pass down through the nozzle ring to a lower portion of the mill housing. At that lower portion there is a conveyor for conveying the material out of the lower portion of the mill housing to an elevator which returns the material to the upper part of the mill housing.